The present invention relates to distortion compensation apparatuses and more particularly, to a distortion compensation apparatus which effectively compensates for a distortion.
As typified by a cellar phone, in these days, such a system as to utilize wireless communication widely spreads. Most of recent wireless systems employ a linear modulation scheme, in which amplification circuits for signals are required to have a linearity feature. As typified by multicarrier amplification, further, signals are amplified at once to reduce a cost and increase an efficiency. In order to satisfy such requirements, an amplification system employing a distortion compensation scheme is generally used, but amplification apparatuses employing various types of compensation schemes have also been examined.
A distortion compensation apparatus compensates for distortion by canceling out a distortion generated in an amplifier. There are two type of methods of compensating for distortion, that is, a method of canceling out a distortion generated in a compensation target amplifier using the generated distortion itself (feedforward scheme, feedback scheme) and a method of canceling out the distortion generated in the compensation target amplifier using an element different from the compensation target amplifier (front-end distortion or predistortion (PD) scheme).
The front-end distortion compensation scheme is divided into an analog type using an analog element and a digital type using such a digital signal processing device as a digital signal processor (DSP).
The feedforward scheme has a merit that a high amount of distortion compensation can be realized with stable distortion compensation, but also has a demerit that a circuit scale becomes large and the cost of the feedforward scheme becomes higher than that of the front-end distortion compensation scheme with a bad efficiency.
Meanwhile, the analog front-end distortion compensation method has a merit that a circuit scale becomes small with a good economy, but also has a demerit that an amount of distortion compensation becomes lower than the compensation amounts of the other distortion compensation methods.
The digital front-end distortion compensation method has a merit that a high amount of distortion compensation can be realized with a good power efficiency and a good productivity, but also has a demerit that a circuit scale becomes large and the cost becomes higher than that of the analog arrangement.
In recent years, the analog front-end distortion compensation scheme having a small circuit scale and a good economy has been reconsidered.